Thermal recording material

ABSTRACT

The present invention provides a thermal recording material excellent in water resistance and oil resistance and satisfactory in wriatbility and stampability. The thermal recording material of the present invention includes a thermally colorable thermal recording layer formed on a substrate and a protective layer formed on said thermal recording layer, in which said protective layer contains a polyvinyl alcohol, chitosan, a crosslinking agent and a pigment, and said pigment contains colloidal silica. Preferably, at least one member selected from an aldehyde-containing compound, a polyamide epichlorohydrin resin or an isocyanate compound is used as said crosslinking agent, and particularly, at least two members selected from an aldehyde-containing compound, a polyamide epichlorohydrin resin and an isocyanate compound are used.

TECHNICAL FIELD

The present invention relates to a thermal recording material comprisinga thermally colorable thermal-recording layer and a protective layerwhich are consecutively formed on a substrate, and particularly to athermal recording material excellent in water resistance and oilresistance and satisfactory in writability and stampability.

TECHNICAL BACKGROUND

Thermal recording materials are available at low prices, recordingdevices for use therewith are simple, printers for use therewith can bedownsized, and their maintenance is easy, so that in recent years theyhave rapidly come to be widely used in the fields of facsimile paper,ATM/CD receipts, receipts for charges for gas, water, electricity, etc.,issued with handy terminals, passenger tickets, coupons, receipts,labels, and the like. While thermal recording materials are sodiversified with regard to fields of use and demands, they are nowrequired to have various properties such as image retainability,high-sensitivity stampability, the running property for recording, andthe like. Particularly, when a thermal recording material is. used witha terminal machine that is used outdoors. such as a handy terminal, orthe like, or in the fields of food labels, etc., the image retainabilityagainst contacts to water or chemicals contained in cosmetics,stationery products, food wrapping materials, etc., is the mostimportant problem to be addressed.

For improving the above thermal recording material in imageretainability, there has been proposed a method in which a protectivelayer having water resistance and oil resistance for preventing theinfiltration of water, oils, plasticizers, etc., is formed on a thermalrecording layer. In the present specification, the “oil resistance”refers to resistance of a thermal recording material against a decreasein recording density and a ground fogging that are caused by the contactof the thermal recording material to oils such as a solvent,plasticizers contained in wrapping materials, human sebum, and the like.

For improving thermal recording materials in abrasion-foggingresistance, retainability, the property of matching to athermal-recording head, etc., for example, there are proposed a methodin which a film of a film-formable polymer is formed on a thermalrecording layer surface (for example, see JP-A-48-051644), a method inwhich an acid-resistant and solvent-resistant protective film is formedon a thermal recording layer surface (for example, see JP-A-54-128347),a method in which a protective film obtained from acarboxyl-group-modified polyvinyl alcohol is formed on a thermalrecording layer surface (for example, see JP-A-56-126193), a method inwhich a protective film obtained from a combination of acarboxyl-group-modified polyvinyl alcohol with a polyamide epoxy resinis formed on a thermal recording layer surface (for example, seeJP-A-59-162088), and a method in which a coating liquid containing anemulsion of composite particles of colloid-formable inorganic silicateand colloidal silica is applied onto a thermal recording layer and theapplied liquid is dried (for example, see JP-A-2-274589).

In recent years, however, recording devices are further decreased insize and enhanced in electric power saving, so that thermal recordingmaterials obtained by the above methods can no longer satisfy the imageretainability against water and chemicals (water resistance and oilresistance) while maintaining the printing sensitivity and the runningproperty for recording.

With regard to a thermal recording material comprising a thermallycolorable thermal recording layer formed on a substrate and a protectivelayer formed on said thermal recording layer, it is proposed that when apolyvinyl alcohol resin, chitosan and an aldehyde compound are used inthe above thermal recording layer or the above protective layer, thereare produced excellent barrier properties (for example, seeJP-A-61-162383). The above thermal recording material has a problem thatit is impaired in gloss, clearness of a printing, water resistance andoil resistance when a pigment is used for improving the material insuitability to a thermal head, writability and stampability. Further,there is also proposed a thermal recording material of which theprotective layer contains chitosan for improving the thermal recordingmaterial in water resistance, oil resistance and matching to a head (forexample, see JP-A-5-572 and JP-A-9-175022). However, the above thermalrecording material is insufficient for attaining both satisfaction ofwater resistance and oil resistance and satisfaction of writability andstampability.

Disclosure of the Invention

It is an object of the present invention to provide a thermal recordingmaterial that overcomes the above defects and that is excellent in waterresistance and oil resistance and is satisfactory in writability andstampability.

The thermal recording material of the present invention is a thermalrecording material comprising a thermally colorable thermal recordinglayer formed on a substrate and a protective layer formed on saidthermal recording layer,

(1) in which said protective layer contains a polyvinyl alcohol,chitosan, a crosslinking agent and a pigment, and said pigment containscolloidal silica.

Further, it is (2) a thermal recording material as recited in (1),wherein as said crosslinking agent is used at least one member selectedfrom an aldehyde-containing compound, a polyamide epichlorohydrin resinand an isocyanate compound.

Further, it is (3) a thermal recording material as recited in (2),wherein as said crosslinking agent are used at least two membersselected from an aldehyde-containing compound, a polyamideepichlorohydrin resin and an isocyanate compound.

Further, it is (4) a thermal recording material as recited in (1),wherein said colloidal silica is cationic colloidal silica.

Further, it is (5) a thermal recording material as recited in (1),wherein said polyvinyl alcohol and said chitosan have a solid contentmass ratio of from 7:3 to 9:1.

Further, it is (6) a thermal recording material as recited in (1),wherein said polyvinyl alcohol is at-least one member selected from anon-modified polyvinyl alcohol having a saponification degree of atleast 95%, a silanol-modified polyvinyl alcohol, an epoxy-modifiedpolyvinyl alcohol, a diacetone-modified polyvinyl alcohol or anacetoacetyl-modified polyvinyl alcohol.

Further, preferably, it is (7) a thermal recording material as recitedin (1), wherein said protective layer contains a water-dispersiblebinder other than any anionic binder.

In the thermal recording material of the present invention, theprotective layer comprises a polyvinyl alcohol, chitosan, a crosslinkingagent and a pigment, and the pigment contains a colloidal silica, sothat the thermal recording material is excellent in water resistance,oil resisance, writability and stampability. Particularly, when analdehyde compound is used as a crosslinking agent, there can be obtaineda thermal recording material excellent in water resistance; when apolyamide epichlorohydrin resin is used as a crosslinking agent, therecan be obtained a thermal recording material excellent in prinabilityand whiteness; and when an isocyanate compound is used as a crosslinkingagent, there can be obtained a thermal recording material excellent inwhiteness. Further, when the polyvinyl alcohol used is at least onemember selected from a non-modified polyvinyl alcohol having asaponification degree of at least 95%, a silanol-modified polyvinylalcohol, an epoxy-modified polyvinyl alcohol, a diacetone-modifiedpolyvinyl alcohol or an acetoacetyl-modified polyvinyl alcohol, therecan be obtained a thermal recording material excellent in waterresistance. Further, when a water-dispersible binder other than anyanionic binder is added, there can be obtained a thermal recordingmaterial excellent in water resistance and stampability, and when thecolloidal silica is increased in average particle diameter, there can beobtained a thermal recording material excellent in stampability andwritability with a pencil.

Best Embodiments for Practicing the Invention

The thermal recording material of the present invention will beexplained in detail hereinafter.

The thermal recording material of the present invention is a thermalrecording material comprising a thermally colorable thermal recordinglayer formed on a substrate and a protective layer formed on saidthermal recording layer, in which said protective layer contains apolyvinyl alcohol, chitosan, a crosslinking agent and a pigment, andsaid pigment contains colloidal silica.

First, the protective layer will be explained.

In the present invention, the polyvinyl alcohol that is incorporatedinto the protective layer includes a non-modified polyvinyl alcohol, andin addition thereto, the polyvinyl alcohol includes acarboxyl-group-modified polyvinyl alcohol, asulfonic-acid-group-modified polyvinyl alcohol, aphosphoric-acid-group-modified polyvinyl alcohol, a silanol-modifiedpolyvinyl alcohol, an epoxy-modified polyvinyl alcohol, adiacetone-modified polyvinyl alcohol and an acetoacetyl-modifiedpolyvinyl alcohol, and also includes a modified polyvinyl alcoholobtained by copolymerization with ethylene, a vinyl ether having along-chain alkyl group, (meth)acrylamide, or the like.

When it is required to further improve the thermal recording material inwater resistance, it is suitable to select at least one member from anon-modified polyvinyl alcohol having a saponification degree of atleast 95%, a silanol-modified polyvinyl alcohol, an epoxy-modifiedpolyvinyl alcohol, a diacetone-modified polyvinyl alcohol and anacetoacetyl-modified polyvinyl alcohol among the above polyvinylalcohols.

The above polyvinyl alcohol works as a binder in the protective layer.

Although being not critical, the polymerization degree of the polyvinylalcohol is generally selected from the range of 100 to 3,000. Further,while the saponification degree is not critical so long as the polyvinylalcohol is soluble in water, it is generally selected from the range of70 to 100 mol %.

In the present invention, chitosan is that which is obtained bydeacetylation of chitin, and it is preferably a product obtained bymodification of at least 50% of acetylamino groups of chitin into aminogroups by deacetylation, and more preferably a product obtained bymodification of at least 90% of them into amino groups by deacetylation.When the chitosan is used, some or all of the amino groups are convertedto ammonium groups with an acid before use. The acid is generallyselected from acetic acid, lactic acid, triphosphoric acid, citric acid,sulfamic acid, hydrochloric acid, sulfuric acid, formic acid, fumaricacid or maleic acid.

Like polyvinyl alcohol, the above chitosan works as a binder in theprotective layer.

While the molecular weight of the chitosan is not critical, preferably,the chitosan has a low molecular weight corresponding to a viscosity of1 to 70 centipoises when an aqueous solution containing 1% by massthereof is measured with a BL type viscometer at 20° C., when thecompatibility thereof with polyvinyl alcohol, etc. is considered.Further, the above chitosan having a low molecular weight has a problemthat it comes to be colored in yellow or brown with the passage of time.In such a case, there may be used a stabilized chitosan which is foundin JP-A-63-72702, or the like.

The content of the polyvinyl alcohol and chitosan in the protectivelayer is preferably 15 to 80 mass % based on the total solid content ofthe protective layer. It is particularly preferably 30 to 80 mass % inview of oil resistance, and it is particularly preferably 15 to 60 mass% in view of writability and stampability.

Further, the solid content mass ratio of the polyvinyl alcohol andchitosan is preferably from 5:5 to 9.7:0.3, more preferably from 7:3 to9:1. When the content of the chitosan is too small, the thermalrecording material is degraded in water resistance. When it is toolarge, there is likely to be caused a problem that a coating. liquid forthe protective layer is increased in viscosity or that the thermalrecording material obtained comes to be colored with the passage oftime.

In the present invention, the protective layer may contain other binderin addition to the polyvinyl alcohol and chitosan, and awater-dispersible binder can be employed as such a binder. The additionof a water-dispersible binder improves the affinity of the thermalrecording material with an oil ink, so that the thermal recordingmaterial can be improved in suitability to printing and stampability. Inthe present invention, the water-dispersible binder refers to a binderobtained by emulsification of a resin with a dispersant, etc. or abinder composed of a self-emulsifiable resin. The water-dispersiblebinder generally includes a styrene/butadiene copolymer, anacrylonitrile/butadiene copolymer, a methyl acrylate/butadienecopolymer, an acrylonitrile/butadiene/styrene terpolymer, polyvinylacetate, a vinyl acetate/acrylic ester copolymer, an ethylene/vinylacetate copolymer, a polyacrylic ester copolymer, a styrene/acrylicester copolymer and polyurethane. In view of stampability,acrylic-ester-based resin is particularly preferred. Further, awater-dispersible binder that is obtained by using an anioic surfactantwhen produced and an anioic water-dispersible binder obtained by partialneutralization of a resin having an acidic group are not preferred inview of liquid properties since chitosan is cationic. For this reason,the water-dispersible binder is.preferably a non-anionicwater-dispersible binder.

The amount of the water-dispersible binder is preferably at least 5 mass% but not more than 60 mass % based on the total binder content of theprotective layer. When it is in the above range, there can be obtainedparticularly preferable oil resistance and affinity with an oil ink.

In the present invention, the crosslinking agent to be contained in theprotective layer includes an aldehyde-containing compound, a polyamideepichlorohdyrin resin, an isocyanate compound (including a blockisocyanate compound, etc.), boric acid, borax, a urea resin, a melamineresin, methylol compounds such as a phenolic resin, etc., epoxycompounds such as a polyfunctional epoxy resin, etc., and oxidants suchas persulfate, peroxide, etc., and it is preferred to use at least onemember selected from an aldehyde-containing compound, a polyamideepichlorohydrin resin or an isocyanate compound.

Of the above crosslinking agents, an adehyde-containing compound isparticularly preferred for improving the thermal recording material inwater resistance. The aldehyde-containing compound includes formalin,glyoxal, etc, and any compound can be used without any speciallimitation so long as it generates an aldehyde in a protective layercoating liquid.

When used as a crosslinking agent, a polyamide epichlorohydrin resin canimprove the thermal recording material in stampability. Further, it canbe also improve the thermal recording material in durability againstyellowing caused by yellow-coloring of chitosan (to be sometimesreferred to as “yellowing durability” hereinafter).

The polyamide epichlorohydrin resin can be obtained by reacting apolyamide resin obtained by condensation of a dicarboxylic acid compoundand a polyalkylene polyamine compound, with epichlorohydrin, forexample, according to a method described in the section of Prior Art inJP-A-9-31192.

The dicarboxylic acid compound includes adipic acid, itaconic acid,malonic acid, succinic acid, sebacic acid, glutaric acid, etc., and thepolyalkylene polyamine compound includes diethyltriamine,triethylenetetramine, and the like.

The polyamide epichlorohydrin resin obtained by reacting a polyamideresin obtained from adipic acid as a dicarboxylic acid compound anddiethylenetriamine as a polyalkylene polyamine compound, withepichlorohydrin is a polymer having a recurring unit represented by thegeneral formula (I).

When an isocyanate compound is used as a crosslinking agent, theyellowing durability can be improved.

The isocyanate compound is not specially limited so long as it is acompound having an isocyanate group or a blocked isocyanate group, andan isocyanate compound having 2 or more isocyanate groups is preferred.Examples of the isocyanate compound include aliphatic isocyanates suchas hexamethylene diisocyanate, aromatic isocyanates such as m-phenylenediisocyanate, p-phenylene diisocyanate, tolyene-2,4-diisocyanate,tolyene-2,6-diisocyanate, diphenylmethane-4,4′-diisocyanate,diphenylene-4,4′-diisocyanate, 4,4′-diisocyanate-3,3′-dimethyldiphenyl,3-methyl-diphenylmethane-4,4′-diisocyanate and diphenyl etherdiisocyanate, and alicyclic isocyanates such ascyclohexane-2,4-diisocyanate, cyciohexane-2,3-diisocyanate andisophorone diisocyanate.

The blocked isocyanate compound included in the isocyanate compound is acompound blocked with a blocking agent such as a phenol-, alcohol-,active methylene-, mercaptane-, amide-, imide- or sulfite-containingblocking agent. For achieving both a pot life and high water resistance,there may be used an isocyanate compound emulsion prepared according toa method found in JP-A-49-96077, or there may be used a commerciallyavailable isocyanate (for example, Aquanate series supplied by NipponPolyurethane Industry Co., Ltd., DURANATE WB series supplied by AsahiChemicals Corporation, DNW-5000 series supplied by Dainippon Ink andChemicals Inc., or the like).

Further, as the above crosslinking agent, it is preferred to use atleast two members selected from an aldehyde-containing compound, apolyamide epichlorohydrin resin and an isocyanate compound.

When the aldehyde-containing compound and the polyamide epichlorohydrinresin are used, there can be obtained a thermal recording material thatachieves both water resistance and stampability. Further, when thealdehyde-containing compound and the isocyanate compound are used, therecan be obtained a thermal recording material that achieves both waterresistance and yellowing durability. Further, when the polyamideepichlorohydrin resin and the isocyanate compound are used, there can beobtained a thermal recording material that achieves both stampabilityand yellowing durability. Furthermore, when the aldehyde-containingcompound, the polyamide epichlorohydrin resin and the isocyanatecompound are used, there can be obtained a thermal recording materialthat is well-balanced among water resistance, stampability and yellowingdurability.

In the present invention, preferably, the amount of the crosslinkingagent to be incorporated into the protective layer is in the range of0.1 to 20 mass % based on the sum total of solid content mass of thepolyvinyl alcohol and chitosan. When it is in the above range, there canbe obtained a protective layer that is stable in water resistance, oilresistance, printing sensitivity and the running property for recording.

The pigment contained in the protective layer contains colloidal silica,and as such colloidal silica, it is preferred to use colloidal silicahaving an average particle diameter in the range of 3 to 200 nm, sinceit has low precipitability in the protective layer coating liquid. Withan increase in the particle diameter of the colloidal silica, thethermal recording material is more improved in the running property forrecording with a thermal recording head, writability and stampability.When high transparency is required, however, the above particle diameteris preferably in the range of 3 to 50 nm. When the colloidal silica isused, the thermal recording material is more improved in the runningproperty for recording with a thermal recording head, writability andstampability without impairing gloss and barrier properties. Generally,the content of the colloidal silica based on the total solid content ofthe protective layer is preferably 5 mass % to 85 mass %, morepreferably 10 mass % to 70 mass %.

Of colloidal silica types, further, cationic colloidal silica is wellcompatible with chitosan and does not easily cause gelling orcoagulation of a protective layer coating liquid. The cationic colloidalsilica refers to colloidal silica in which at least silica particlesurfaces are cationically charged since a compound of polyvalent metalion such as aluminum ion or the like or an organic cationic compound iscontained on silica surfaces or inside each particle. Of cationiccolloidal silica types, colloidal silica cationized with basic aluminumis particularly preferred.

In the present invention, as a pigment, a conventionally known pigmentmay be used in a small amount in addition to the colloidal silica. Thepigment can be selected from inorganic pigments such as diatomite, talc,kaolin, calcined kaolin, calcium bicarbonate, precipitable calciumcarbonate, chalk, magnesium carbonate, zinc oxide, aluminum oxide,aluminum hydroxide, magnesium hydroxide, titanium dioxide, bariumsulfate, zinc sulfate, amorphous silica, amorphous calcium silicate,etc., and organic pigments such as a melamine resin filler, aurea-formalin resin filler, a polyethylene powder, a nylon powder, andthe like. Of these, aluminum hydroxide is effective for improving athermal recording head in prevention of abrasion, and amorphous silicais effective for the thermal recording material in prevention ofsticking to a thermal recording head.

For improving the running properties such as prevention of the abrasionof a thermal recording head, prevention of sticking to a thermalrecording head, and the like, the protective layer contains, asrequired, higher fatty acid metal salts such as zinc stearate and thelike, higher fatty acid amides such as stearamide and the like, andwaxes such as paraffin, polyethylene wax, polyethylene oxide, castor waxand the like.

In the present invention, the solid content coating amount of theprotective layer is preferably 0.3 to 10 g/m². When it is in the aboverange, there can be obtained a protective layer excellent in waterresistance, oil resistance and printing sensitivity. As a layerconstitution, the protective layer may be a single layer or amulti-layer so long as the coating amount is in the above range.

The thermal recording layer will be explained below.

In the present invention, materials to be contained in the thermalrecording material are not specially limited, and any materials can beused so long as a combination thereof causes a coloring reaction bymeans of energy applied by a thermal recording head. For example, theabove combination includes a combination of a colorless or light-coloredelectron-donating dye precursor with an electron-accepting developer anda combination of an aromatic isocyanate compound with an imino compound.

While the colorless or light-colored electron-donating dye precursor foruse in the thermal recording layer is typified by those dye precursorswhich are used in generally pressure-sensitive recording papers andthermal recording papers, there is no special limitation to be imposedthereon. Specific examples thereof include

(1) triarylmethane compounds:3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide (Crystal VioletLactone), 3,3-bis(p-dimethylaminophenyl) phthalide,3-(p-dimethylaminophenyl)-3-(1,2-dimethylindol-3-yl) phthalide,3-(p-dimethylaminophenyl)-3-(2-methylindol-3-yl) phthalide,3-(p-dimethylaminophenyl)-3-(2-phenylindol-3-yl) phthalide,3,3-bis(1,2-dimethylindol-3-yl) -5-dimethylaminophthalide,3,3-bis(1,2-dimethylindol-3-yl)-6-dimethylaminophthalide,3,3-bis(9-ethylcarbazol-3-yl)-5-dimethylaminophthalide,3,3-bis(2-phenylindol-3-yl)-5-dimethylaminophthalide,3-p-dimethylaminophenyl-3-(1-methylpyrol-2-yl)-6-dimethylaminophthalide,etc.,

(2) diphenylmethane compounds;4,4′-bis(dimethylaminophenyl)benzhydrylbenzyl ether,N-chlorophenylleucoauramine, N-2,4,5-trichlorophenylleucoauramine, etc.

(3) xanthene compounds: rhodamine B anilinolactam, rhodamineB-p-chloroanilinolactam, 3-diethylamino-7-benzylaminofluorane,3-diethylamino-7-octylaminofluorane, 3-diethylamino-7-phenylfluorane,3-diethylamino-7-chlorofluorane,3-diethylamino-6-chloro-7-methylfluorane,3-diethylamino-7-(3,4-dichloroanilino)fluorane,3-dibutylamino-7-(2-chloroanilino)fluorane,3-diethylamino-7-(2-chloroanilino)fluorine,3-diethylamino-6-methyl-7-anilinofluorane,3-dibutylamino-6-methyl-7-anilinofluorane,3-dipentylamino-6-methyl-7-anilinofluorane, 3-(N-ethyl-N-tolyl)amino-6-methyl-7-anilinofluorane,3-piperidino-6-methyl-7-anilinofluorane,3-(N-ethyl-N-tolyl)amino-6-methyl-7-phenethylfluorane,3-diethylamino-7-(4-nitroanilino)fluorine,3-dibutylamino-6-methyl-7-anilinofluorane,3-(N-methyl-N-propyl)amino-6-methyl-7-anilinofluorane,3-(N-ethyl-N-isoamyl)amino-6-methyl-7-anilinofluorane,3-(N-methyl-N-cyclohexyl)amino-6-methyl-7-anilinofluorane,3-(N-ethyl-N-tetrahydrofurfuryl)amino-6-methyl-7-anilinofluorane,3-diethylamino-6-methyl-7-(3-trifluoromethylanilino)fluorane, etc.,

(4) thiazine compounds: benzoylleucomethylene blue,p-nitrobenzoylleucomethylene blue, etc., and

(5)spiro compounds; 3-methylspirodinaphthopyran,3-ethylspirodinaphthopyran, 3,3′-dichlorospirodinaphthopyran,3-benzylspirodinaphthopyran, 3-methylnaphtho-(3-methoxybenzo)spiropyran,3-propylspirobenzopyran, etc. These dye precursors may be used singly orin combination.

The electron-accepting compound for use in the thermal recording layeris generally typified by acidic substances, and it is selectedparticularly from phenol derivatives, aromatic carboxylic acidderivatives, N,N′ -diarylthiourea derivatives, polyvalent metal saltssuch as zinc salt of an organic compound, or the like.

Specifically, the electron-accepting compound includes known substances;diphenylsulfone derivatives such as4-hydroxy-4′-isopropoxydiphenylsulfone,4-hydroxy-4′-n-propoxydiphenylsulfone, 4,4′-dihydroxydiphenylsulfone,2,4′-dihydroxydiphenylsulfone, 4-hydroxydiphenylsulfone,-4-hydroxy-4′-methyldiphenylsulfone, 4-hydroxy-4′-methoxydiphenylsulfone,4-hydroxy-4′-ethoxyphenylsulfone, 4-hydroxy-4′-n-butoxydiphenylsulfone,4-hydroxy-4′-benzyloxydiphenylsulfone,bis(3-allyl-4-hydroxyphenyl)sulfone,bis(3,5-dibromo-4-hdyroxyphenyl)sulfone,bis(3,5-dichloro-4-hydroxyhenyl)sulfone, 3,4-dihydroxydiphenylsulfone,3,4-dihydroxy-4′-methyldiphenylsulfone,3,4,4,′-trihydroxydiphenylsulfone, 3,4,3′,4′-tetrahydroxydiphenylsulfone, 2,3,4-trihydroxydiphenylsulfone,3-phenylsulfonyl-4-hydroxydiphenylsulfone,2,4-bis(phenylsulfonyl)phenol, etc., 4-tert-butylphenol,2,2′-dihydroxydiphenyl, 2,2′-methylenebis(4-methyl-6-tert-butylphenol),4,4′-ethylenebis(2-methylphenol),4,4′-cyclohexylidenebis(2-isopropylphenol), p-phenylphenol,p-hydroxyacetophenone, 1,1-bis(p-hydroxyphenyl)propane,1,1-bis(p-hydroxyphenyl)pentane, 1,1-bis(p-hydroxyphenyl)hexane,1,1-bis(p-hydroxyphenyl)cyclohexane, 2,2-bis(p-hydroxyphenyl)propane,2,2-bis(p-hydroxyphenyl)hexane, 1,1-bis(p-hydroxyphenyl)-2-ethylhexane,2,2-bis(3-chloro-4-hydroxyphenyl)propane,1,1-bis(p-hydroxyphenyl)-1-phenylethane,1,3-di-[2-(p-hydroxyphenyl)-2-propyl]benzene,1,3-di[2-(3,4-dihydroxyphenyl)-2-propyl]benzene,1,4-di-[2-(p-hydroxyphenyl)-2-propyl]benzene, 4,4′-hydroxydiphenylether, 3,3′-dichloro-4,4′-hydroxyphenyl sulfide, bis(4-hydroxyphenyl)acetic esters such as methyl 2,2-bis(4-hydroxyphenyl)acetate, butyl2,2-bis(4-hydroxyphenyl)acetate, etc.,4,4′thiobis(2-tert-butyl-5-methylphenol), dimethyl 4-hydroxyphthalate,benzoic acid, hydroxybenzoic esters such as ethyl p-hydroxybenzoate,propyl p-hydroxybenzoate, butyl p-hydroxybenzoate, benzylp-hydroxybenzoate, etc., benzyl gallate, stearyl gallate,N,N′diphenylthiourea,4,4′bis(3-(4-methylphenylsulfonyl)ureiodo)diphenylmethane,N-(4-methyl-phenylsulfonyl)-N′phenylurea, salicylanilide,5-chlorosalicylanilide, salicylic acid, 3-isopropyl salicylic acid,3-cyclohexyl salicylic acid, 3,5-di-tert-butyl salicylic acid,3,5-di-α(-methylbenzyl salicylic acid,4-[2′(4-methoxyphenoxy)ethyloxy]salicylic acid,3-(octyloxycarbonylamino)salicylic acid, metal salts of these salicylicacid derivatives, gallic acid alkyl esters, phenolic compounds such as anovolak type phenolic resin, etc., naphthoic acid derivatives such as1-hydroxy-2-naphthoic acid, 2-hydroxy-6-naphthoic acid, etc., and metalsalts of these, organic acids such as tartaric acid, oxalic acid, boricacid, citric acid, stearic acid, etc., N-(p-toluenesulfonyl)-N′(3-p-toluenesulfonyloxyphenyl)urea,N-(4-hydroxyphenyl)-p-toluenesulfonamide,N-(4-hydroxyphenyl)benzenesulfonamide,N-(4-hydroxyphenyl)-1-naphthalenesulfonamide,N-(4-hydroxyphenyl)-2-naphthalenesulfonamide,N-(4-hydroxynaphthyl)-p-toluenesulfonamide,N-(4-hydroxynaphthyl)benzenesulfonamide,N-(4-hydroxynaphthyl)-1-naphthalenesulfonamide,N-(4-hydroxynaphthyl)-2-naphthalenesulfonamide,N-(3-hydroxyphenyl)-p-toluenesulfonamide,N-(3-hydroxyphenyl)benzenesulfonamide,N-(3-hydroxyphenyl)-1-naphthalenesulfonamide,N-(3-hydroxyphenyl)-2-naphthalenesulfonamide, and the like. These may beused singly or in combination as required.

The aromatic isocyanate compound for use in the thermal recording layeris a colorless or light-colored compound that is a solid at roomtemperature. Specifically, the aromatic isocyanate compound includes2,6-dichlorophenyl isocyanate, p-chlorophenyl isocyanate, 1,3-phenylenediisocyanate, 1,4-phenylene diisocyanate,1,3-dimethylbenzene-4,6-diisocyanate,1,4-dimethylbenzene-2,5-diisocyanate, 1-ethoxybenzene-2,4-diisocyanate,2,5-dimethoxybenzene-1,4-diisocyanate,2,5-diethoxybenzene-1,4-diisocyanate,2,5-dibutoxybenzene-1,4-diisocyanate, azobenzene-4,4′diisocyanate,diphenyl ether-4,4′-diisocyanate, naphthalene-1,4-diisocyanate,naphthalene-1,5-diisocyanate, naphthalene-2,6-diisocyanate,naphthalene-2,7-diisocyanate, 3,3′dimethylbiphenyl-4,4′-diisocyanate,3,3′dimethoxy-4,4′diisocyanate, diphenylmethane-4,4′diisocyanate,diphenyldimethylmethane-4,4′diisocyanate,benzophenone-3,3′-diisocyanate, fluorine-2,7-diisocyanate,anthraquinone-2,6-diisocyanate, 9-ethylcarbazole-3,6-diisocyanate,pyrene-3,8-diisocyanate, naphthalene-1,3,7-triisocyanate,biphenyl-2,4,4′-triisocyanate,4,4′,4″-triisocyanate-2,5-dimethoxytriphenylamine, p-dimethylaminophenylisocyanate, tri(4-phenyl isocyanate)thiophosphate, and the like. Thesemay be used singly or in combination as required.

These aromatic isocyanate compounds may be used in the form of so-calledblock isocyanates that are addition compounds thereof with phenols,lactams, oximes, etc, and may be used in the form of dimers such as adimer of 1-methylbenzene-2,4-diisocyanate or trimers such asisocyanurate, or they may be also used as polyisocyanates formed byadding them with various polyols.

The imino compound for use in the thermal recording layer is a colorlessor light-colored compound that is a solid at a room temperature.Specifically, the imino compound includes3-imino-4,5,6,7-tetrachloroisoindoline-1-one,1,3-diimino-4,5,6,7-tetrachloroisoindoline, 1,3-diiminoisoindoline,1,3-diiminobenz(f)isoindoline, 1,3-diiminonaphtho(2,3-f)isoindoline,1,3-dimino-5-nitroisoindoline, 1,3-diimino-5-phenylisoindoline,1,3-diimino-5-methoxyisoindoline, 1,3-diimino-5-chloroisoindoline,5-cyano-1,3-diiminoisoindoline, 5-acetamido-1,3-diiminoisoindoline,1,3-diimino-5-(1H-1,2,3-triazol-1-yl)-isoindoline,5-(p-tert-butylphenoxy)-1,3-diiminoisoindoline, 5-(p-cumylphenoxy)-1,3-diiminoisoindoline,5-isobutoxy-1,3-diiminoisoindoline,1,3-diimino-4,7-dimethoxyisoindoline,4,7-diethoxy-1,3-diiminoisoindoline,4,5,6,7-tetrabromo-1,3-diiminoisoindoline,4,5,7-trichloro-1,3-diimino-6-methylmercaptoisoindoline, 1-iminodiphenicacid imide, 1-(cyano-p-nitrophenylmethylene)-3-iminoisoindoline,1-(cyanobenzothiazolyl-(2′)-carbamoylmethylene)-3-iminoisoindoline,1-[(cyanobenzimidazolyl-2′)methylene]-3-iminoisoindoline,1-[(cyanobenzimidazolyl-2′)-methylene]-3-imino-4,5,6,7-tetrachloroisoindoline,1-[cyanobenzimidazolyl]-2′)-methylene]-3-imino-5-methoxyisoindoline,1-[(l′phenyl-3′methyl-5-oxo) -pyrazolidene-4′]-3-iminoisoindoline,3-imino-l-sulfobenzoic acid imide,3-imino-l-sulfo-4,5,6,7-tetrachlorobenzoic acid imide,3-imino-1-sulfo-4,5,7-trichloro-6-methylmercaptobenzoic acid imide,3-imino-2-methyl-4,5,6,7-tetrachloroisoindolin-1-one, and the like.These may be used singly or in combination as required.

In the thermal recording material of the present invention, the thermalrecording layer may contain a heat-melting substance for improving itsthermal response. The heat-melting substance preferably has a meltingpoint of 60° C. to 180° C., particularly preferably has a melting pointof 80° C. to 140° C. Specifically, the heat-melting substance includesknown heat-melting substances; stearic acid amide,N-hydroxymethylstearic acid amide, N-hydroxymethylstearic acid amide,N-stearylstearic acid amide, ethylenebisstearic acid amide,N-stearylurea, β-naphthylbenzyl ether, m-terphenyl, 4-benzylbiphenyl,2,2′bis(4-methoxyphenoxy)diethyl ether, α, α′-diphenoxyxylyene,bis(4-methoxyphenyl)ether, 1,2-di(3-methylphenoxy)ethane,1,2-diphenoxyethane, diphenyl adipate, oxalic diester derivatives suchas dibenzyl oxalate, di(4-chlorobenzyl) oxalate, di(4-methylbenzyl)oxalate, etc., sulfone compounds such as diphenyl sulfone, dimethylterephthalate, dibenzyl terephthalate, phenyl benzenesulfonate,bis(4-allyloxyphenyl) sulfone, 4-acetylacetophenone, acetoacetic acidanilides, fatty acid anilides, and the like. These compounds may be usedsingly or in combination as required. Further, preferably, the contentof the heat-melting substance in the total solid content of the thermalrecording layer is 5 to 50 mass % for obtaining sufficient thermalresponse.

The binder for use in the thermal recording layer can be selected fromvarious binders that are used for general coating. Specifically, thebinder includes water-soluble binders such as starches, hydroxymethylcellulose, methyl cellulose, ethyl cellulose, carboxymethyl cellulose,gelatin, casein, polyvinyl alcohol, modified polyvinyl alcohol, sodiumalginate, polyvinyl pyrolidone, polyacrylamide, an acrylamide/acrylatecopolymer, an acrylamide/acrylate/methacrylic acid terpolymer, an alkalisalt of polyacrylic acid, an alkali salt of polymaleic acid, an alkalisalt of a styrene/maleic anhydride copolymer, an alkali salt of anethylene/maleic anhydride copolymer, an alkali salt of anisobutylene/maleic anhydride copolymer, etc., and water-dispersiblebinders such as a styrene/butadiene copolymer, anacrylonitrile/butadiene copolymer, a methyl acrylate/butadienecopolymer, an acrylonitrile/butadiene/styrene terpolymer, polyvinylacetate, a vinyl acetate/acrylate copolymer, an ethylene/vinyl acetatecopolymer, polyacrylic ester, a styrene/acrylate copolymer,polyurethane, etc., while the binder shall not be limited to these.These may be used singly or in combination as required.

In addition to these, the thermal recording layer may contain, as apigment, inorganic pigments such as diatomite, talc, kaolin, calcinedkaolin, calcium bicarbonate, precipitable calcium carbonate, chalk,magnesium carbonate, zinc oxide, aluminum oxide, aluminum hydroxide,magnesium hydroxide, titanium dioxide, barium sulfate, zinc sulfate,amorphous silica, amorphous calcium silicate, colloidal silica, etc., orand organic pigments such as a melamine resin filler, a urea-formalinresin filler, a polyethylene powder, a nylon powder, and the like.

The thermal recording layer optionally contains higher fatty acid metalsalts such as zinc stearate, calcium stearate, etc., lubricants such asparaffin, polyethylene wax, polyethylene oxide, castor wax, etc.,ultraviolet absorbents such as a benzophenone-containing ultravioletabsorbent, a benzotriazole-containing ultraviolet absorbent, etc., andan anionic or nonionic surfactant (including a high-molecular-weightsurfactant), and may further optionally contains a fluorescent dye, ananti-foaming agent, and the like as required.

In the present invention, thermal recording layer is obtained by mixingaqueous dispersions in which colorable components are dispersed in afinely milled state, with a binder, etc., applying the mixture onto asubstrate and drying the applied mixture. The thermal recording layermay have a single-layer or multi-layered constitution.

The application amount for the thermal recording layer is suitably suchthat the application amount of a dye precursor solid content isgenerally in the range of 0.1 to 2.0 g/m2. When it is in the aboverange, economically advantageously, a sufficient recording density canbe obtained.

In the present invention, paper is mainly used as a substrate. Inaddition to paper, the substrate can be selected from various wovenfabrics, non-woven fabrics, synthetic resin films, synthetic resinlaminated papers, metal foils, deposition sheets, or composite sheetsobtained by combining these by laminating, as required.

The thermal recording material of the present invention may be provided,as required, with an intermediate layer formed of a single layer or aplurality of layers between the thermal recording layer and theprotective layer and at least one undercoat layer formed of a singlelayer or a plurality of layers formed from a pigment or a resin betweenthe substrate and the thermal recording layer.

The intermediate layer includes, for example, a layer formed from aresin and a crosslinking agent as described in JP-A-59-45191 and a layercontaining an ultraviolet absorbent as described in JP-A-7-179045 orJP-A-2000-185472.

When the thermal recording material of the present invention has anundercoat layer, the application amount of a solid content for theundercoat layer is preferably 1 to 30 g/m, more preferably 3 to 20 g/m2.

Calcined kaolin is generally used as a pigment for the undercoat layer.Besides it, the pigment can be selected from inorganic pigments such asdiatomite, talc, kaolin, calcium bicarbonate, precipitable calciumcarbonate, chalk, magnesium carbonate, zinc oxide, aluminum oxide,aluminum hydroxide, magnesium hydroxide, titanium dioxide, bariumsulfate, zinc sulfate, amorphous silica, amorphous calcium silicate,colloidal silica, etc., or and organic pigments such as a melamine resinfiller, a urea-formalin resin filler, a polyethylene powder, a nylonpowder, and the like. Further, organic spherical particles, organichollow particles, etc., can be also used.

The binder for the undercoat layer can be selected from variouswater-soluble resins or water-dispersible resins that are used ingeneral coating. For example, the binder includes water-soluble resinssuch as starches, hydroxymethyl cellulose, methyl cellulose, ethylcellulose, carboxymethyl cellulose, gelatin, casein, polyvinyl alcohol,modified polyvinyl alcohol, sodium alginate, polyvinyl pyrolidone,polyacrylamide, an acrylamide/acrylate copolymer, anacrylamide/acrylate/methacrylic acid terpolymer, an alkali salt ofpolyacrylic acid, an alkali salt of polymaleic acid, an alkali salt of astyrene/maleic anhydride copolymer, an alkali salt of an ethylene/maleicanhydride copolymer and an alkali salt of an isobutylene/maleicanhydride copolymer, etc., and water-dispersible binders such as astyrene/butadiene copolymer, an acrylonitrile/butadiene copolymer, amethyl acrylate/butadiene copolymer, an acrylonitrile/butadiene/styreneterpolymer, polyvinyl acetate, a vinyl acetate/acrylate copolymer, anethylene/vinyl acetate copolymer, a polyacrylic ester copolymer, astyrene/acrylate copolymer, polyurethane, and the like.

The thermal recording material of the present invention can be obtainedby consecutively forming the thermal recording layer and protectivelayer on the substrate. The thermal recording layer may be formed afterformation of the undercoat layer on the substrate as required, and theintermediate layer may be formed after formation of the thermalrecording layer.

The method for forming each of the thermal recording layer, theprotective layer, the intermediate layer and the undercoat layer is notspecially limited, and they can be formed according to known methods. Asa specific embodiment, there can be employed a constitution in whicheach coating liquid is applied by the method of air knife coating, rodblade coating, bar coating, blade coating, gravure coating, curtaincoating, E bar coating or the like and then each applied coating liquidis dried, to form the thermal recording layer, the protective layer, theintermediate layer and the undercoat layer. Further, super calenderingmay be carried out after formation of the undercoat layer, afterformation of the thermal recording layer, after formation of theintermediate layer or after formation of the protective layer, forimproving the thermal recording material in image quality.

The present invention will be explained with reference to Exampleshereinafter, while the present invention shall not be limited by theseExamples. In Examples, “%” and “part” are both based on weight.

(Preparation of Dispersions)

Dispersions A, B, C, D, E and F were prepared according to the followingmethods.

(Dispersion A)

200 Grams of 3-dibutylamino-6-methyl-7-anilinofluorane was dispersed ina mixture of 200 g of a 10% polyvinyl alcohol aqueous solution with 600g of water and milled with a bead mill until it had an average particlediameter of 1 μm.

(Dispersion B)

400 Grams of 2,2-bis(p-hydroxyphenyl)propane was dispersed in a mixtureof 400 g of a 10% polyvinyl alcohol aqueous solution with 200 g of waterand milled with a bead mill until it had an average particle diameter of1 μm.

(Dispersion C) 400 Grams of β-naphthyl benzyl ether was dispersed in amixture of 400 g of a 10% polyvinyl alcohol aqueous solution with 200 gof water and milled with a bead mill until it had an average particlediameter of 1 μm.

(Dispersion D)

200 Grams of preciptable calcium carbonate was dispersed in 800 g of a0.5% sodium polyacrylate aqueous solution and dispersed with a homomixerfor 10 minutes.

(Dispersion E)

200 Grams of amorphous silica ((Mizukasil P527, supplied by MizusawaIndustrial Chemicals, Ltd.) was dispersed in 800 g of a 0.5% sodiumpolyacrylate aqueous solution and dispersed with a homomixer for 10minutes.

(10% Chitosan Aqueous Solution)

400 Grams of water was added to 50 g of chitosan (OTS-2, supplied byKuraray Co., Ltd.), 50 g of 50% lactic acid was added with stirring, andthe mixture was further stirred to prepare a 10% chitosan aqueoussolution.

(50% Block Isocyanate Aqueous Solution)

6.8 Parts of hexamethylene diisocyanate and 8.2 parts of sodiummetabisulfite were dissolved in 15 parts of water, and the mixture wassealed and stirred for 20 hours to prepare a block isocyanate aqueoussolution.

EXAMPLE 1

<Thermal Recording Layer>

Materials including Dispersions A to D were mixed in the followingamount ratio, and the mixture was fully stirred to prepare a thermalrecording layer coating liquid. Dispersion A 20 parts Dispersion B 15parts Dispersion C 15 parts Dispersion D 25 parts 10% Polyvinyl alcoholaqueous solution 30 parts Water 30 parts

The thus-prepared thermal recording layer coating liquid was applied toa base paper having a basis weight of 40 g/m2 such that the solidcontent coating amount of a dye precursor was 0.3 g/m2, and the appliedcoating liquid was dried and then super calendered to obtain a materialhaving a thermal recording layer.

<Protective Layer>

Materials were mixed in the following amount ratio, and the mixture wasfully stirred to prepare a protective layer coating liquid. 10%Completely saponified polyvinyl alcohol aqueous 80 parts solution(PVA-1177, supplied by Kuraray Co., Ltd.) 10% Chitosan aqueous solution20 parts 25% Glyoxal aqueous solution 3.2 parts  20% Cationic colloidalsilica aqueous dispersion 15 parts (SNOWTEX AK, supplied by NissanChemical Industries, Ltd., average particle diameter 10 to 20 nm) 40%Zinc stearate aqueous solution  2 parts Water 80 parts

A protective layer coating liquid prepared in the above amount ratio wasapplied to the above-obtained thermal recording layer so as to have asolid content coating amount of 2 g/m², and the applied coating liquidwas super calendered to give a thermal recording material.

EXAMPLE 2

A thermal recording material was obtained in the same manner as inExample 1 except that the 25% glyoxal aqueous solution in theformulation of the protective layer in Example 1 was replaced with a 25%polyamide epichlorohydrin resin (WS-547, supplied by Seiko PMCCorporation) aqueous solution having the same amount as that of the 25%glyoxasole aqueous solution.

EXAMPLE 3

A thermal recording material was obtained in the same manner as inExample 1 except that 3.2 parts of the 25% glyoxal aqueous solution inthe formulation of the protective layer in Example 1 was replaced with1.6 parts of a 50% block isocyanate aqueous solution and 1.6 parts ofwater.

EXAMPLE 4

A thermal recording material was obtained in the same manner as inExample 1 except that 3.2 parts of the 25% glyoxal aqueous solution inthe formulation of the protective layer in Example 1 was replaced with1.6 parts of a 25% glyoxal aqueous solution and 1.6 parts of a 25%polyamide epichlorohydrin resin (WS-547, supplied by Seiko PMCCorporation) aqueous solution.

EXAMPLE 5

A thermal recording material was obtained in the same manner as inExample 1 except that 3.2 parts of the 25% glyoxal aqueous solution inthe formulation of the protective layer in Example 1 was replaced with1.6 parts of a 25% glyoxal aqueous solution, 0.8 part of a 50% blockisocyanate aqueous solution and 0.8 part of water.

EXAMPLE 6

A thermal recording material was obtained in the same manner as inExample 1 except that 3.2 parts of the 25% glyoxal aqueous solution inthe formulation of the protective layer in Example 1 was replaced with1.6 parts of a 25% polyamide epichlorohydrin resin (WS-547, supplied bySeiko PMC Corporation) aqueous solution, 0.8 part of a 50% blockisocyanate aqueous solution and 0.8 part of water.

EXAMPLE 7

A thermal recording material was obtained in the same manner as inExample 1 except that 3.2 parts of the 25% glyoxal aqueous solution inthe formulation of the protective layer in Example 1 was replaced with16 parts of a 5% boric acid aqueous solution and that the amount ofwater was changed from 80 parts to 67.2 parts.

EXAMPLE 8

A thermal recording material was obtained in the same manner as inExample 1 except that the amount of the 10% completely saponifiedpolyvinyl alcohol aqueous solution in the formulation of the protectivelayer in Example 1 was changed from 80 parts to 60 parts and that theamount of the 10% chitosan aqueous solution in the formulation of theprotective layer was changed from 20 parts to 40 parts.

EXAMPLE 9

A thermal recording material was obtained in the same manner as inExample 1 except that the amount of the 10% completely saponifiedpolyvinyl alcohol aqueous solution in the formulation of the protectivelayer in Example 1 was changed from 80 parts to 95 parts and that theamount of the 10% chitosan aqueous solution in the formulation of theprotective layer was changed from 20 parts to 5 parts.

EXAMPLE 10

A thermal recording material was obtained in the same manner as inExample 1 except that the 20% cationic colloidal silica aqueousdispersion in the formulation of the protective layer in Example 1 wasreplaced with a 20% anionic colloidal silica aqueous dispersion (SNOWTEXC, supplied by Nissan Chemical Industries, Ltd., average particlediameter 10 to 20 nm)

EXAMPLE 11

A thermal recording material was obtained in the same manner as inExample 1 except that 80 parts of the 10% completely saponifiedpolyvinyl alcohol aqueous solution in the formulation of the protectivelayer in Example 1 was replaced with 80 parts of a 10% partiallysaponified polyvinyl alcohol aqueous solution (PVA-217, supplied byKuraray Co., Ltd., saponification degree 88%).

Thermal Recording Material

EXAMPLE 12

A thermal recording material was obtained in the same manner as inExample 1 except that 80 parts of the 10% completely saponifiedpolyvinyl alcohol aqueous solution in the formulation of the protectivelayer in Example 1 was replaced with 80 parts of a 10% silanol-modifiedpolyvinyl alcohol aqueous solution (R-1130, supplied by Kuraray Co.,Ltd.).

EXAMPLE 13

A thermal recording material was obtained in the same manner as inExample 1 except that 80 parts of the 10% completely saponifiedpolyvinyl alcohol aqueous solution in the formulation of the protectivelayer in Example 1 was replaced with 80 parts of a 10% epoxy-modifiedpolyvinyl alcohol aqueous solution (W100, supplied by Denki Kagaku KogyoK.K.).

EXAMPLE 14

A thermal recording material was obtained in the same manner as inExample 1 except that 80 parts of the 10% completely saponifiedpolyvinyl alcohol aqueous solution in the formulation of the protectivelayer in Example 1 was replaced with 80 parts of a 10%diacetone-modified polyvinyl alcohol aqueous solution (D1700, suppliedby The Shin-Etsu Chemical Co., Ltd.).

EXAMPLE 15

A thermal recording material was obtained in the same manner as inExample 1 except that 80 parts of the 10% completely saponifiedpolyvinyl alcohol aqueous solution in the formulation of the protectivelayer in Example 1 was replaced with 80 parts of a 10%acetoacetyl-modified polyvinyl alcohol aqueous solution (Z200, suppliedby Nippon Synthetic Chemical Industry Co., Ltd.).

EXAMPLE 16

A thermal recording material was obtained in the same manner as inExample 13 except that the amount of the 25% glyoxal aqueous solution inthe formulation of the protective layer in Example 13 was changed from3.2 parts to 6 parts and that 1.6 parts of a 25% polyamideepichlorohydrin resin (WS-547, supplied by Seiko PMC Corporation)aqueous solution was added in the formulation of the protective layer.

EXAMPLE 17

A thermal recording material was obtained in the same manner as inExample 1 except that 15 parts of the 20% cationic colloidal silicaaqueous dispersion in the formation of the protective layer in Example 1was replaced with 10 parts of a 30% large-particle-diameter cationiccolloidal silica aqueous dispersion. (SNOWTEX AK-YL, supplied by NissanChemical Industries, Ltd., average particle diameter 70 to 80 nm) and 5parts of water.

EXAMPLE 18

A thermal recording material was obtained in the same manner as inExample 1 except that 80 parts of the 10% completely saponifiedpolyvinyl alcohol aqueous solution and 20 parts of the 10% chitosanaqueous solution in the formulation of the protective layer in Example 1were replaced with 64 parts of a 10% completely saponified polyvinylalcohol aqueous solution, 16 parts of a 10% chitosan aqueous solutionand 6.7 parts of a 30% nonionic acryl emulsion (Biniplan 2685, suppliedby The Shin-Etsu Chemical Co., Ltd.).

EXAMPLE 19

A thermal recording material was obtained in the same manner as inExample 18 except that 64 parts of the 10% completely saponifiedpolyvinyl alcohol aqueous solution in the formulation of the protectivelayer in Example 18 was replaced with 64 parts of a 10% epoxy-modifiedpolyvinyl alcohol aqueous solution (W100, supplied by Denki Kagaku KogyoK.K.) and that 3.2 parts of the 25% glyoxal aqueous solution in theformulation of the protective layer in Example 18 was replaced with 1.6parts of a 25% glyoxal aqueous solution, 0.8 part of a 50% blockisocynate aqueous solution and 0.8 part of water.

EXAMPLE 20

A thermal recording material was obtained in the same manner as inExample 19 except that 15 parts of the 20% cationic colloidal silicaaqueous dispersion in the formation of the protective layer in Example19 was replaced with 10 parts of a 30% large-particle-diameter cationiccolloidal silica aqueous dispersion (SNOWTEX AK-YL, supplied by NissanChemical Industries, Ltd., average particle diameter 70 to 80 nm) and 5parts of water.

COMPARATIVE EXAMPLE 1

A thermal recording material was obtained in the same manner as inExample 1 except that 15 parts of the 20% cationic colloidal silicaaqueous dispersion in the formation of the protective layer in Example 1was replaced with 15 parts of Dispersion E.

COMPARATIVE EXAMPLE 2

A thermal recording material was obtained in the same manner as inExample 1 except that 15 parts of the 20% cationic colloidal silicaaqueous dispersion in the formation of the protective layer in Example 1was replaced with 15 parts of water.

COMPARATIVE EXAMPLE 3

A thermal recording material was obtained in the same manner as inExample 1 except that 20 parts of the 10% chitosan aqueous solution inthe formulation of the protective layer in Example 1 was replaced with20 parts of a 10% completely saponified polyvinyl alcohol.

COMPARATIVE EXAMPLE 4

A thermal recording material was obtained in the same manner as inExample 1 in the formation of the protective layer in Example 1 exceptthat 3.2 parts of the 25% glyoxal aqueous solution was replaced with 3.2parts of water.

COMPARATIVE EXAMPLE 5

A thermal recording material was obtained in the same manner as inExample 1 except that 20 parts of the 10% chitosan aqueous solution inthe formation of the protective layer in Example 1 was replaced with 10parts of a 20% anionic acryl emulsion (OM1050, supplied by MitsuiChemicals, Inc.) and 10 parts of water.

COMPARATIVE EXAMPLE 6

A thermal recording material was obtained in the same manner as inExample 1 except that no protective layer was formed.

The protective layer coating liquids prepared in the above examples 1 to20 and Comparative Examples 1 to 6 were evaluated for liquid properties,and the thermal recording materials obtained were tested as follows.Table 1 shows the results.

(1) Coating Liquid Property

Prepared protective layer coating liquids were evaluated for liquidproperties with the passage of time. The valuation was made according tothe following ratings.

⊚: A coating liquid was free of gelling and coagulation and permittedcoating without any problem.

∘: While a coating liquid permitted coating, it showed an increase inviscosity to some extent or showed coagulation to some extent.

Δ: While a coating liquid permitted coating, it showed an increase inviscosity or coagulation.

X: A coating liquid greatly increased in viscosity or coagulated, sothat no coating was permitted.

(2) Printing Sensitivity

Printing was carried out on the obtained thermal recording materialswith a facsimile tester TH-PMD supplied by Ohkura Electric Co., Ltd. Athermal head having a dot density of 8 dots/mm and a head resistance of1681 Ω was used, and the tester was electrically powered at a headvoltage of 23 V at a pulse width of 1.0 msec. Each image was measuredfor a color density with a Macbeth RD-918 reflection densitometer. Inthe five-stage evaluation, a larger value shows that a thermal recordingmaterial has higher printing sensitivity, and it was judged that thermalrecording materials rated at 3 or more were at a practical use level.

(3) Plasticizer Resistance

A color-formed surface of each of. thermal recording materials on whichprinting was carried out under the conditions in (2) was intimatelycontacted to a commercially available soft vinyl chloride film, and theywere left in a 40° C. and 90% high-temperature chamber for 2 days. Then,a printed portion on each thermal recording material was measured for acolor density, followed by comparisons. The thermal recording materialswere evaluated according to the following ratings.

⊚: There is almost no change in density of a printed portion after thetesting.

∘: The density in a printed portion decreases to some extent after thetesting.

Δ: The density in a printed portion decreases to a great extent afterthe testing.

X: Almost no printed portion remains after the testing.

(4) Water Resistance

Five milliliters of water was dropped on the thermal recording materialson which printing was carried out under conditions in (2), followed byrubbing with a finger for 10 seconds. The thermal recording materialswere evaluated according to the following ratings.

⊚: There is almost no change in a protective layer, and there is nochange in a printed portion, either.

∘: While a protective layer peels off to some extent, there is almost nochange in a printed portion.

Δ: While a protective layer peels off, there is almost no change in aprinted portion.

X: A protective layer completely peels off, and a printed portion alsopeels off.

(5) Writability with Pencil

Characters were written on each of the obtained thermal recordingmaterials, and the thermal recording materials were evaluated forwritability with a pencil. The thermal recording materials wereevaluated according to the following ratings.

⊚: Characters are writable without any problem, and the characters areclear.

∘: While characters were writable, the characters are less clear to someextent.

Δ: Characters are light in density.

X: Written characters are hard to recognize.

(6) Stampability

Printing was carried out on thermal recording materials with a dye-basedink X stamper supplied by Shachihata Inc., and after 10 seconds, a drycloth was wiped over a stamped portion, followed by-evaluation forremaining properties of stamped characters. The thermal recordingmaterials were evaluated according to the following ratings.

⊚: Stamped characters mostly remain.

∘: While stamped characters becomes light in density to some extent,they are clearly recognizable.

Δ: While satamped characters becomes light in density to some extent,they are somehow recognizable.

X: Stamped characters are not recognizable.

(7) Whiteness

Obtained thermal recording materials were left in an atmosphere at 60°C. for 1 week, and measured for whiteness, followed by comparisons withthose obtained before the treatment. The thermal recording materialswere evaluated according to the following ratings.

⊚: There is almost no change from the whiteness obtained before thetreatment.

602 :There is caused a decrease in whiteness to such an extent that thedecrease is not recognizable with the eyes.

Δ: There is caused a decrease in whiteness to such an extent that thedecrease is somehow recognizable with the eyes. TABLE 1 CoatingWritability liquid Thermal Plasticizer Water with property printingresistance resistance pencil Stampability Whiteness Example 1 ◯ 4 ⊚ ◯ ◯◯ ◯ Example 2 ◯ 4 ⊚ Δ ◯ ⊚ ⊚ Example 3 ◯ 4 ⊚ Δ ◯ ◯ ⊚ Example 4 ◯ 4 ⊚ ◯ ◯⊚ ⊚ Example 5 ◯ 4 ⊚ ◯ ◯ ◯ ⊚ Example 6 ◯ 4 ⊚ Δ ◯ ⊚ ⊚ Example 7 ◯ 4 ⊚ Δ ◯◯ ⊚ Example 8 Δ 4 ⊚ ⊚ ◯ ◯ Δ Example 9 ◯ 4 ◯ Δ Δ Δ ⊚ Example 10 Δ 4 ◯ ◯ ◯◯ ◯ Example 11 ◯ 4 ⊚ Δ ◯ ◯ ◯ Example 12 ◯ 4 ⊚ ⊚ ◯ ◯ ◯ Example 13 ◯ 4 ⊚ ⊚◯ ◯ ◯ Example 14 ◯ 4 ⊚ ⊚ ◯ ◯ ◯ Example 15 Δ 4 ⊚ ⊚ ◯ ◯ ◯ Example 16 ◯ 4 ⊚⊚ ◯ ⊚ ⊚ Example 17 ◯ 4 ⊚ ◯ ⊚ ⊚ ◯ Example 18 ◯ 4 ⊚ ⊚ ◯ ⊚ ⊚ Example 19 ◯ 4⊚ ⊚ ◯ ⊚ ⊚ Example 20 ◯ 4 ⊚ ⊚ ⊚ ⊚ ⊚ CEx. 1 X 3 X ◯ ⊚ ⊚ ◯ CEx. 2 ◯ 4 Δ ◯ XX ⊚ CEx. 3 ◯ 4 Δ X X X ⊚ CEx. 4 ◯ 4 ⊚ X ◯ ◯ ⊚ CEx. 5 X 3 X Δ ◯ ⊚ ⊚ CEx.6 — 5 X X Δ ⊚ ⊚CEx. = Comparative Example

As is clear from Table 1, when compared with Example 1 using chitosan,it is seen that Comparative Examples 3 and 5 using no chitosan areinferior in water resistance, and particularly that Comparative Example5 using an anionic acryl emulsion is poor in coating liquid property,water resistance and plasticizer durability. Further, when Example 1using colloidal silica is compared with Comparative Examples 1 and 2using no colloidal silica, it is seen that Comparative Example 1 is poorin coating liquid property and plasticizer durability, and thatComparative Example 2 is poor in plasticizer durability, writabilitywith a pencil and stampability.

Further, when Examples 1 to 3 and 7 are compared with ComparativeExample 4, it is seen that glyoxal is the most effective as acrosslinking agent for improvments in water resistance, it is seen thata polyamide epichlorohydrin resin or block diisocynate is excellent inimprovements in whiteness, and it is also seen that the stampability isimproved when a polyamide epichlorohydrin resin is used.

When Example 4 is compared with Comparative Examples 1 to 3, or whenExample 13 is compared with Example 16, it is seen that the waterresistance and the stampability can be satisfied at the same time byusing glyoxal and a polyamide epichlorohydrin resin as crosslinkingagents, when Example 5 is compared with Examples 1 to 3, it is seen thatthe water resistance and the whiteness can be satisfied together byusing glyoxal and block isocyanate, and when Example 6 is compared withExamples 1 to 3, it is seen that the whiteness and the stampability canbe satisfied together by using a polyamide epichlorohydrin resin andblock isocyanate.

Further, when Example 1 is compared with Comparative Example 10, it isseen that a protective layer coating liquid free from gelling andcoagulation can be prepared by using cationic colloidal silica as acolloidal silica, and that the thus-obtained thermal recording materialis excellent in plasticizer durability.

Further, when Example 1 is compared with Examples 8 and 9, it is seenthat the coating liquid property and the water resistance can besatisfied together when the amount ratio of polyvinyl alcohol andchitosan by mass is 8:2 in Example 1, that while the water resistance isimproved, the liquid property of a coating liquid is poor due to alittle increase in viscosity when it is 6:4 in Example 8, and that thewater resistance, the writability with a pencil and the stampability arepoor as compared with Example 1 when it is 9.5:0.5 in Example 9.

Further, when Example 1 is compared with Example 11, it is seen that acompletely saponified polyvinyl alcohol serves to accomplish excelelntwater resitance over a partially saponified polyvinyl alcohol. WhenExample 1 is compared with Examples 12 to 15, or when Example 4 iscompared with Example 16, it is seen that the use of at least one memberselected from silanol-modified polyvinyl alcohol, epoxy-modifiedpolyvinyl alcohol, diacetone-modified polyvinyl alcohol oracetoacetyl-modified polyvinyl alcohol serves to accomplish excellentwater resistance over completely saponified polyvinyl alcohol.

Further, when Exmple 1 is compared with Example 18, it is seen thatExample 18 in which the protective layer contains a nonionic acrylemulsion is improved in water resistance and stampability over Example 1in which the protective layer does not contain the same.

Further, when Example 1 is compared with Example 17, or when Example 19is compared with Example 20, it is seen that the writability with apencil is more improved with an increase in the average particlediameter of colloidal silica. Further, when Example 1 is compared withExample 17, it is seen that the stampability is more improved with anincrease in the average particle diameter of colloidal silica.

INDUSTRIAL UTILITY

According to the present invention, there can be provided a thermalrecording material that is excellent in water resistance and oilresistance and that is satisfactory in writability and stampability.

1. A thermal recording material comprising a thermally colorable thermalrecording layer formed on a substrate and a protective layer formed onsaid thermal recording layer, said protective layer containing apolyvinyl alcohol, chitosan, a crosslinking agent and a pigment, andsaid pigment containing colloidal silica.
 2. The thermal recordingmaterial of claim 1, wherein as said crosslinking agent is used at leastone member selected from an aldehyde-containing compound, a polyamideepichlorohydrin resin or an isocyanate compound.
 3. The thermalrecording material of claim 2, wherein as said crosslinking agent areused at least two members selected from an aldehyde-containing compound,a polyamide epichlorohydrin resin and an isocyanate compound.
 4. Thethermal recording material of claim 1, wherein said colloidal silica iscationic colloidal silica.
 5. The thermal recording material of claim 1,wherein said polyvinyl alcohol and said chitosan have a solid contentmass ratio of from 7:3 to 9:1.
 6. The thermal recording material ofclaim 1, wherein said polyvinyl alcohol is at least one member selectedfrom a non-modified polyvinyl alcohol having a saponification degree ofat least 95%, a silanol-modified polyvinyl alcohol, an epoxy-modifiedpolyvinyl alcohol, a diacetone-modified polyvinyl alcohol or anacetoacetyl-modified polyvinyl alcohol.
 7. The thermal recordingmaterial of claim 1, wherein said protective layer contains awater-dispersible binder other than any anionic binder.